This invention relates to circuits especially adapted for operating gaseous discharge lamps and in particular, to circuits of the electronic ballast type. A wide range of such circuits has been considered in the past. While the basic high frequency oscillator electronic ballast circuit is well known, a variety of problems have been encountered, including cost, efficiency, power factor, power consumption, lamp output control with power supply variation and lamp aging and start-up, and protection against open circuits and lamp burn-out, and provision for dimming of lamp output.
It is an object of the present invention to provide a new and improved electronic ballast circuit, sometimes referred to as a switched mode electronic ballast. A particular object is to provide such a circuit which inherently generates starting pulses for the gaseous discharge tubes, and one which can be utilized to start and operate a single high intensity discharge or fluorescent tube or a plurality of tubes, and when utilized with a plurality of tubes, independent starting may be obtained for each tube.
It is another object to provide a circuit which is suitable for use with continuous and/or emergency lighting applications, and in particular a circuit which may be used for uninterrupted lighting applications.
It is an object to provide an electronic ballast circuit which operates at high frequency, typically 20,000 hertz, though the switching frequency is not critical, with the circuit providing for dimming of lamp output and eliminating lamp flicker.
It is an object to provide an electronic ballast circuit in which the operation is regulated by feedback circuitry to maintain the lamp output constant while supply voltage changes and while the lamp ages. A further object is to provide such a circuit which provides protection for the circuit components during lamp start-up, lamp burn-out and operation in the absence of a lamp or other load.
It is an object also to provide a power supply high frequency switched mode filter to enable unity power factor loading of the 60 hertz line and full wave rectifier and to vastly reduce the peak current required to be supplied by the 60 hertz full wave rectifier compared to that which would be required by a conventional capacitor-input 120 hertz ripple filter. This switched mode filter can, in a typical situation, be operated synchronously with the switched mode electronic ballast (tube driver) referred to above.
Other objects, advantages, features and results will more fully appear in the course of the following description.